R1:9c3949379617

Make WorkingCopyBlueprint responsible for performing merges

Summary:
Ref T182. Currently, the "RepositoryLand" operation is responsible for performing merges when landing a revision.

However, we'd like to be able to perform these merges in a larger set of cases in the future. For example:

- After Releeph is revamped, when someone says "I want to merge bug fix X into stable branch Y", it would probably be nice to make that a Buildable and let tests run against it without requring that it actually be…